Top lieutenants of terrorist Abu Musab al-Zarqawi, and possibly the chief of Al-Qaeda in Iraq himself, met in Syria about a month ago to plot more insurgent attacks in Iraq, a senior US military official said.

The meeting, prompted by Al-Zarqawi's anger over a lull in terror strikes, led to the spike in violence that has left hundreds dead -- including an Iraqi general killed in a driveby shooting Wednesday -- the military official said.

Al-Zarqawi "wasn't happy with how the insurgency was going. The government was getting stronger and coalition forces not being defeated," the official said, according to accounts of the briefing, which was given on the condition that he not be named.

The Syrian meeting has led to one of the bloodiest periods since the US-led invasion two years ago. Nearly 500 people have been killed since the country's new Shia-dominated government was announced April 28.

Several Shia and Sunni Muslim clerics were among the victims, raising fears that sectarian tensions could ignite a civil war.

A chilling, rambling Internet audiotape purportedly made by Al-Zarqawi denounced Iraq's Shias as US collaborators and said killing them is justified.

"God ordered us to attack the infidels by all means ... even if armed infidels and unintended victims -- women and children -- are killed together," the speaker said. The tape could not immediately be authenticated.

The Syrian foreign and information ministries were unavailable for comment on the alleged terrorist gathering on their soil. Iraq's presidential adviser for security affairs, Gen. Wafiq al-Samarie, said he had "no information" about an Al-Zarqawi meeting in Syria.

In response to Al-Zarqawi's order, there have been 21 car bombings, mostly suicide attacks, in Baghdad during May, compared with 25 such attacks in all of 2004, said the senior military official. Nearly 130 car bombs have exploded or been defused since late February, he said.